Seismic velocity changes in the Groningen reservoir associated with distant drilling

In this study, we show that passively recorded data of nearby passing trains by a deep borehole geophone array could be linked to fluctuations of the gas-water contact in the Groningen reservoir in The Netherlands. During a period of 1.5 months, changes of inter-geophone P-wave travel times were detected by deconvolution interferometry of the recorded train signals. P-to-S converted waves, obtained by deconvolution of the horizontal component by the vertical component at individual geophones, showed simultaneous variations. The observed travel-time changes could be related to fluctuations of the gas-water contact in the observation well caused by pressure variations at a well drilling 4.5 km away. The \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim$$\end{document}∼ 3.5 day delay between drilling in the reservoir and the seismic response yields a hydraulic diffusivity of approximately 5 m\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^2$$\end{document}2/s and suggests that the pressure front is effectively propagated over such a long distance. Our observations illustrate that downhole geophone arrays can be used to monitor changes in the subsurface if repeating noise sources are available, and that unexpected effects may occur due to drilling.

The stacks are for ∼30 trains per day in each direction. Figure S3: P-wave travel times from stacked deconvolutions per day for all geophone combinations. Rows show diagrams for geophones acting as virtual source, columns those for geophones acting as virtual receiver. The first deployment period (Jan 23 -Jun 29, 2015) has a lightgrey background, the second deployment period (Jul 3 -1 Dec 1, 2015) is in darker grey. Travel-time shifts between the two deployments are caused by small changes in geophone locations. P-wave travel times obtained for trains from Stedum to Loppersum are in blue, for Loppersum to Stedum they are in red. Travel times along the vertical axis are in ms.
2 Time-lapse borehole fluid wave travel times by deconvolution interferometry Apart from the low-amplitude train signals (30 -90 Hz), the geophone array also recorded intermittently occurring, high-amplitude (> 4 µm /s), high-frequency (15 -1000 Hz) downward propagating waves (Fig. S4). Their signals are without a clear onset and the waves have an apparent velocity close to 1500 m/s, the speed of an acoustic wave in water. Figure S4: Example of a high-frequency borehole fluid wave.
The source of these waves is not clear and their occurrence is irregular (see below). However, their characteristics, with a propagation speed close to that of an acoustic wave in water, their high-frequency content and the absence of dispersion, suggest that they are acoustic waves that travel within the borehole fluid. Borehole fluid waves, as we call them, have previously been detected by (11)  Similar to the train signals, we applied deconvolution interferometry to the borehole fluid waves. The deconvolution was calculated on band-pass filtered signals (100 -300 Hz) and a 200-Hz low-pass filter was applied after that. Finally, the deconvolved responses were stacked per day to improve the signalto-noise ratio.   3 Time-lapse P-to-S converted wave travel times by Horizontal-Vertical deconvolution of train signals

Horizontal-Vertical deconvolution: HZdecon
The receiver function method has been widely used by the seismological community to image subsurface discontinuities, mostly by employing incident P waves from teleseismic earthquakes (8). A (Pwave) receiver function is the deconvolution of the horizontal component by the vertical component for a time window that encompasses the direct P wave with a coda that includes near-receiver P-to-S converted waves (6; 5). It eliminates the source wavelet of the incoming wave and the common travel time to the interface at which conversion takes place. Contrary to the standard receiver function setting with P-wave incidence from below (from teleseismic earthquakes) and a receiver at the surface, in the cur- where Z * (ω) is the complex conjugate of Z(ω) and Φ(ω) the vertical-component auto-correlation with a water level tuned by the parameter c.
G(ω) is the Gaussian low-pass filter determined by frequency α, After transformation back into the time domain the HZdecons show PS (P-to-S converted) waves as arrivals at delay times relative to the direct P wave.

Synthetic HZdecons
To illustrate the method, we first applied the method to synthetic data and we then compared the results to the observed train-signal HZdecons. Here we give a summary, more details can be found in (12).  Figure S7: (a) Seismic velocity and density model derived from sonic log data of SDM-1 (green). Nomenclature of stratigraphic layers adopted from (7) and (9) The sonic log data of well SDM-1, provided by NAM, were smoothed by harmonic averaging over a 60 m sliding window to obtain a realistic, yet relatively smooth, P-wave velocity model from the surface down to the gas reservoir. The corresponding S-wave velocity and density profiles were computed from the P-wave velocities with the relations given by (7) for the various lithologies. The model is shown in

Time-lapse train-signal HZdecons
To identify potential time-lapse changes in the HZdecons, we stacked the responses per day (∼30 trains) for trains traveling in the two opposite directions. The results showed no significant travel time shifts, except for those of geophone 10 for the period from mid July to the beginning of September (see Fig.  S9a for trains from Stedum to Loppersum). 4 Noise level and micro-earthquake . The data of geophones 3, 5 and 9 are not shown because geophone 9 was out of order, as well as the horizontal components of geophones 3 and 5.

P-wave velocity above and below GWC
The figure below shows how the average P-wave velocity above and below the GWC was determined.
The region between the dashed and solid red line (GWC) is ignored because it is the zone with reduced hydrocarbon saturation.  Figure S11: Determination of the average P velocity above and below the GWC from sonic log data. The region between the dashed and solid (GWC) red line is excluded because there is a gradual change in gas saturation just above the GWC.